1. Field of the Invention
The present invention relates to a water heating apparatus and method utilizing passive flue gas recirculation for reducing NO.sub.x emissions during heating.
2. Description of the Prior Art
Water heaters and boilers form nitrogen oxides during combustion. The high combustion temperatures typical of such devices fix some oxides of nitrogen. These combustion compounds are found in flue gases mainly as nitric oxide (NO), with lesser amounts of nitrogen dioxide (NO.sub.2) and other oxides. The total amount of nitric oxide plus nitrogen dioxide in a flue gas effluent is referred to simply as nitrogen oxides, or NO.sub.x.
Depending on the type of fuel, two types of nitrogen oxide reactions occur. In the first type of reaction, fuel bound NO.sub.x forms from nitrogen present in the fuel itself, for example, fuel oils. During combustion, nitrogen released from the fuel reacts rapidly with oxygen from the combustion air to form NO.sub.x. These fuel bound reactions are not particularly temperature-dependent. In the second type of reaction, thermal NO.sub.x forms at high combustion temperatures. High combustion temperatures break down the nitrogen gas in air to atomic nitrogen. The atomic nitrogen subsequently reacts with oxygen to form thermal NO.sub.x.
Nitrogen oxide emissions are air pollutants. Various state and federal agencies regulate the amount of NO.sub.x in vented gases, especially in heavily populated areas such as the Los Angeles Basin of California. Tightening state and federal regulations for emission requirements warrant the effort to find new ways to remove or prevent the formation of nitrogen oxides in combustion processes to avoid further harmful effects on the environment.
The type of fuel burned affects the type and amount of NO.sub.x. If only natural gas is used, thermal NO.sub.x is formed exclusively, because natural gas does not contain any nitrogen containing compounds. The burning of fuel oils, on the other hand, forms both thermal and fuel bound NO.sub.x. No. 6 oil, for example, contains large amounts of nitrogen and thus produces a large amount of fuel bound NO.sub.x.
It is well known that cooling the combustion flame temperature decreases NO.sub.x production. The effect of flame temperature reduction decreases thermal NO.sub.x production with a lesser effect on decreasing fuel bound NO.sub.x production. Therefore, the flame temperature reduction by the recirculation of flue gas is most effective when burning natural gas.
Flue gas recirculation reduces NO.sub.x emissions from water heating systems by decreasing the amount of NO.sub.x formed. Typically, a duct connects a flue stack to a recirculation fan. Another duct couples the fan to the combustion air inlet of a burner or the combustion chamber. Since these systems directly feed the recirculated flue gas to the burner flame region where fuel is also being introduced, they often require the use of control devices to regulate the feed of recirculated flue gas for efficient and safe combustion.
U.S. Pat. No. 4,545,329, issued Oct. 8, 1985, and assigned to the assignee of the present invention, describes a unique submerged combustion chamber/forced draft burner water heater having improved efficiency characteristics. Optimizing NO.sub.x reduction in such water heating devices using submergible, pressurized combustion chambers and high turbulence power burners, presents problems not encountered in other conventional water heating systems. For example, such systems differ from other water heating systems by the amount of injected combustion air and by pressure drops at various locations in the system. The preferred fire tube apparatus described in U.S. Pat. No. 4,545,329 requires much higher air injection pressures to force combustion gases through the constricted, narrow fire tubes which help to increase efficiency of the unit. This results in shorter residence time for the flue gases in the fire tubes. Thus, greater recirculation rates are required for a fire tube type apparatus to obtain adequate NO.sub.x reduction.
To the best of Applicant's knowledge, prior attempts to use passive flue gas recirculation have not been successful in systems of the type presently under consideration. In this disclosure, the term "passive" refers to employing no additional active components, such as fans, impellers, blowers, control devices, and the like, other than the components that are already a part of the heating apparatus. These passive systems either do not work for a submergible, pressurized combustion device with a high turbulence power burner, often due to poor combustion, or the level of NO.sub.x emissions is too high for the present government regulations. For example, many power burners with air suction and high pressure delivery characteristics cannot use a passive recirculation system. These systems require a separate blower strictly for the purpose of inducing combustion products from the burner vent and forcing them into a combustion chamber where fuel is also being introduced. In the process, combustion air and recirculated flue gas incompletely mixes before introducing the fuel, thus leading to incomplete combustion.
Prior passive systems do not mix fresh air with the recirculated flue gas prior to combustion. These systems blow recirculated flue gas directly into the burner flame where it remains separated from combustion air.
The present invention has one object, to produce a passive flue gas recirculation system for a submerged combustion chamber/forced draft burner water heating device that reduces NO.sub.x levels, yet does not interfere with the efficiency of combustion.
Another object of the present invention is to produce a water heating device having a passive flue gas recirculation system which premixes flue gases and fresh air in a gas-mixing region of the burner prior to the introduction of fuel and the beginning of the combustion process.
Another object of the invention is to produce such a water heating device having NO.sub.x emissions below about 30 to 40 ppm and a heating efficiency of at least about 83%.
Another object of the present invention is to produce such a device which is relatively simple in design and economical to manufacture.
Another object of the present invention is to eliminate the complexity and failure modes associated with non-passive, or active NO.sub.x flue gas recirculation control systems.